LENGTH-DISPLACEMENT SCALING AND THE FORMATION OF UPHILL-FACING LUNAR THRUST FAULTS

The vertical displacements along four lunar thrust-fault scarps (Simpelius-1, Morozov (S1), Fowler, Racah X-1) range in length from 1.3 km to 15.4 km [1]. The maximum total displacements of the four thrust faults follow a linear increase with length over one order of magnitude. We interpret this relationship to indicate that during the progressive accumulation of slip, lunar faults propagate laterally and increase in length. For the studied faults, the ratio of maximum displacement, D, to fault length, L, ranges from 0.017 to 0.028 with a mean value of ~2.3%. Our results and other recently published findings for other lunar scarps [2,3], indicate that the D/L ratios of lunar thrust faults are similar to those on Mercury and Mars [4-7], and almost as high as the average D/L ratio of ~3% for faults on Earth [8,9]. Three of the studied thrust fault scarps (Simpelius-1, Morozov (S1), Fowler) are uphill-facing scarps generated by slip on faults that dip in the same direction as the local topography. Thrust faults with such geometry are quite common (~60% of 97 studied scarps) on the Moon [e.g., 1,2,10]. To test our hypothesis that the surface topography controls the vertical load on a fault plane and, hence, plays an important role in the formation of uphill-facing fault scarps, we simulated thrust faulting and its relation to topography with two-dimensional finite-element models. Our model results indicate that the onset of faulting in our 200-km-long model is a function of the surface topography. We modeled two faults, i.e., one uphill-facing and one downhill-facing scarp. The model results demonstrate that, for all other factors being equal, the differing weight of the hanging wall above the two model faults is responsible for the different timing of fault initiation, as well as the difference in total slip. To illustrate the state of stress that controls the initiation of faulting for different topographic slopes, we present a Mohr circle analysis of the problem.

[1] Roggon et al. (2017) Icarus; [2] Williams et al., 2013, JGR. 118; [3] Banks et al., 2013, LPSC 44; [4] Watters et al., 2000, GRL 27 [5] Hauber and Kronberg, 2005, JGR 110; [6] Hauber et al., 2013, EPSC2013-987; [7] Byrne et al., 2014, Nature Geosci. 7; [8] Schlische et al., 1996, Geology 24; [9] Torabi and Berg, 2011, Marine Petrol. Geol. 28; [10] Banks et al., 2012, JGR 117.